This invention relates to preforms for optical fibers and, in particular, to a method of preparing preforms for silicic optical fibers.
Preforms are used as a parent material for silicic optical fibers. Two methods dominate the field of preform manufacture. These methods are: (1) VAD method, and (2) MCVD method.
In the VAD method, developed by Nippon Telephone and Telegram, a material gas, such as silicon tetrachloride (SiCl.sub.4) or germanium tetrachloride (GeCl.sub.4), is transferred over an oxygen/hydrogen burner to yield fine particles of glass. The fine particles of glass are attached to the end of a seed bar. This is accomplished by spraying the fine particles so that they grow into a porous preform in the direction of the axis of the seed bar. Once the preform is prepared, it is heated so that it becomes transparent.
According to the MCVD method, developed by Bell Laboratories, a material gas, such as silicon tetrachloride (SiCl.sub.4) or germanium tetrachloride (GeCl.sub.4), is blown using oxygen gas to form a silica glass tube. The glass tube is heated from the outside in order to carry out the reaction and to yield fine particles of glass. The fine particles of glass are attached to the inner surface of the tube so that the tube no longer has a center hole. Then the openings at the end of the tube are closed by heating and a transparent preform is obtained.
Although the VAD and MCVD methods for preparing preforms are most commonly used, other methods are known. These include the OVD method, POD method and sol-gel method.
These conventional methods of preparing preforms for optical fibers have many disadvantages. For example, all of the VAD, MCVD, OVD and POS methods are gas phase and involve the chemical reaction of the gas material. For this reason, they have a low reaction yield and low productivity. Furthermore, it is difficult to obtain a preform having a desired refractive index distribution. Additionally, the manufacturing machines required are expensive and consequently, the preforms are also expensive.
While using the sol-gel method makes it possible to provide a glass at a significantly lower cost than can be provided using the gas phase methods, it is difficult to provide a glass article that is large enough to be used as a preform for optical fibers. An example of a sol-gel method is disclosed in Japanese Laid-Open Publication No. 55/100231 of Hitachi Co., Ltd.
Accordingly, it is desirable to provide a method for preparing preforms for optical fibers of larger dimension and higher quality than those which can be provided using prior art methods and which overcomes the disadvantages inherent in the prior art.